Electric fish barriers have been used for decades to constrain the movement of fish in waterways such as rivers and reservoirs. Exemplary is U.S. Pat. No. 4,750,451 (incorporated herein by reference), in which electrodes extend across the bottom of a river and are pulsed synchronously to prevent fish from migrating upstream. The amplitude of pulses applied to each electrode can be controlled independently, permitting a user to tailor a desired field gradient profile to approaching fish. In the illustrated embodiment, the potential difference between adjacent electrodes increases as the fish moves upstream (i.e. voltages of 0 v., 100 v., 300 v., 600 v., 1000 v.--a so-called "graduated field").
While the foregoing arrangement utilizes horizontally oriented electrodes, arrangements utilizing vertically oriented electrodes are also known. In one such system 10, shown in FIG. 1a, a plurality of elongated electrodes (e.g. pipes) 12 are positioned in a river, forming a "curtain array" 14. In this system, a power supply 16 (either A.C. or D.C.) provides a single potential difference, with alternating electrodes being driven with the same voltage. (In this and all succeeding embodiments, it should be understood that the electrodes are not in direct contact with the ground beneath the water.)
The foregoing arrangement has a number of disadvantages. One of the most serious is that the resulting electrical field does not extend far from the array. Instead, the electrical field lines are concentrated close to the array's axis, with the orientation of the electric field between the electrodes. Consequently, fish encounter the barrier with little or no forewarning and are thus often injured or killed when they suddenly encounter the strong fields extending directly between electrodes. Further, electric fields are most effective when they have a component extending lengthwise of the fish (its greatest dimension). In the FIG. 1 arrangement, the field lines traverse the fish essentially from side to side.
FIG. 1b shows a system having an electrode configuration like that of FIG. 1a, but driven with three-phase AC. This arrangement overcomes some of the problems of the FIG. 1 array but introduces others (i.e. AC fish barriers are generally more stressful and tend to kill and injure fish).
In FIG. 2, another curtain array barrier 18 is shown. In this system, however, all of the vertical electrodes 20 are driven with the same polarity signal. A second electrode 22 (usually positioned on the bottom) is driven with the other polarity and provides the requisite potential difference.
While generally an improvement over the FIG. 1 system, the FIG. 2 arrangement still suffers, inter alia, by reason of the electrical field geometry. While spacing the second electrode 22 away from the vertical electrodes 20 causes the electrical field to extend out from the he curtain array, the field is highly depth-dependent. Further, the field is still largely confined to the region between the cooperating electrodes.
FIG. 3a shows yet another prior art arrangement. In this system, a plurality of vertical electrodes are disposed on each side of a waterway. Those on one side are driven from one terminal of the power supply, those on the other side are driven with the other, providing a potential difference (again A.C. or D.C.) across the waterway. This system has many of the same drawbacks as those noted earlier.
A variant, shown in FIG. 3b, has the rows of electrodes disposed across, rather than alongside, the waterway.
U.S. Pat. No. 5,078,542 shows a fish barrier associated with an intake watercourse to a power plant or the like. The walls of the watercourse are inclined outwardly and have electrodes formed therein. A power supply provides a potential difference between the electrodes on one wall and those on the other.
Like the foregoing arrangements, the system described in this patent suffers, inter alia, by reason of its field geometry. As with the other systems, the field fails to penetrate much beyond the region directly between the cooperating electrodes. As a consequence, fish have little warning of the barrier until they are suddenly exposed to a relatively strong field. Further, the field lines are oriented 90 degrees away from the optimal orientation (i.e. traversely across the fish rather than lengthwise).
While the foregoing discussion has focused on electrical fish barriers, it should be mentioned that other barrier mechanisms have been tried as well. U.S. Pat. No. 2,826,897, for example, shows a louver-based barrier used to screen fish from turbine intakes in a dam or the like. The disclosed louvers create localized turbulence which the fish swim to avoid. The fish are thereby channeled to a relatively small waterway bypassing the turbine intakes. This patent also discloses the use of trash racks to screen waterborne debris (e.g. logs) from the louvers.
While louver-based barriers are well suited for some applications, they are ill-suited for many others. As noted, one problem is their tendency to clog with waterborne debris. Another is that they are inefficient with small fish.
In its different aspects, the present invention seeks to remedy the above-described and other drawbacks of the prior art, and to provide advantages not heretofore contemplated.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.